An increased carbon dioxide (CO2) density in the atmosphere has proved to be one of major factors of global warming which has recently come into attention as a global-scale environmental problem. A thermal power plant appears close-up as a fixed source of exhausting these substances. Fuel for thermal power generation may be oil, natural gas and coal, among which coal is especially anticipated to have a large future demand due to its greater potential reserves.
Coal contains a higher percentage of carbon as compared with natural gas and oil, together with other components such as hydrogen, nitrogen and sulfur and ash as an inorganic component. Therefore, when coal is burned in the air, most of the composition of the exhaust gas is occupied by nitrogen (about 70%), with the remainder occupied by carbon dioxide CO2, sulfur oxide SOx, nitrogen oxide NOx, oxygen (about 4%), unburned combustibles and particles such as ash. The exhaust gas is thus subjected to exhaust gas treatments such as denitration, desulfurization and dedusting so that NOx, SOx and particulates fall under their respective environmental emission standard values before the emission to the atmosphere through a stack.
NOx occurring in the exhaust gas divides into a thermal NOx generated from oxidization of nitrogen in the air by oxygen and a fuel NOx generated as a result of oxidization of nitrogen in the fuel. Up until now, a combustion method of lowering the flame temperature has been employed for reduction of the thermal NOx, whereas another combustion method of forming a fuel-excess region for deoxidizing NOx within a boiler has been employed for reduction of the fuel NOx.
In case of using a fuel such as coal containing sulfur, a wet or dry desulfurizing device has been provided to remove SOx occurring in the exhaust gas as a result of the combustion.
It is desired on the other hand that a large amount of carbon dioxide generated in the exhaust gas be also separated and captured at a high efficiency. A possible method of capturing carbon dioxide contained in the exhaust gas has hitherto been reviewed which includes a method of using an amine or other absorbing liquid to absorb it, an adsorption method of using a solid adsorbent to adsorb it or a membrane separation method, all of which have a low conversion efficiency, thus not yet reaching a practical use level of the CO2 capture from a coal burning boiler.
Accordingly, a technique of burning a fuel with oxygen instead of air has been proposed as an effective technique to address at one time both the problem of the separation of carbon dioxide in the exhaust gas and the problem of the reduction of the thermal NOx (see, e.g., Patent Literatures 1 and 2).
When coal is burned with oxygen, generation of the thermal NOx is not seen and most of the exhaust gas is occupied by carbon dioxide with the remainder occupied by other gases containing the fuel NOx, SOx and unburned combustibles, consequently achieving a relatively easy liquefaction and separation of the carbon dioxide through cooling of the exhaust gas.
[Patent Literature 1] JP 5-231609A
[Patent Literature 2] JP 3053914B